A wide variety of coatings have been used to coat the surfaces of packaging articles (e.g., food and beverage cans). For example, light metal packaging cans are sometimes coated using “coil coating” or “sheet coating” operations, i.e., a planar coil or sheet of a suitable substrate (e.g., steel or aluminum metal) is coated with a suitable composition and hardened (e.g., cured). The coated substrate then is formed into the can end or body. Alternatively, liquid coating compositions may be applied (e.g., by spraying, dipping, rolling, etc.) to the formed article or components thereof and then hardened (e.g., cured).
Packaging coatings should preferably be capable of high-speed application to the substrate and provide the necessary properties when hardened to perform in this demanding end use. For example, the coating should be safe for food contact, have excellent adhesion to the substrate, and resist degradation over long periods of time, even when exposed to harsh environments and/or packaged products.
Phenolic resins have been used widely in coatings used in food and beverage containers as well as in other coating applications. Phenolic resins offer many benefits. In addition to being relatively low cost, phenolic resins protect substrates against staining, corrosion, moisture damage, acid damage, alkaline damage, and other chemical damage. They also impart coating hardness and protect substrates against physical damage. Phenolic resins also can be used as crosslinking agents in combination with other thermosetting resins having complementary curing functionality.
Phenolic resins have been most widely used in coating compositions in which the coating solids are dispersed in organic liquid carriers with only a minor amount of water, if any. Yet, water-based coating compositions are in high demand. A water-based composition uses a liquid carrier with a significant amount of water, e.g., at least 5 weight percent, even at least 20 weight percent, or even at least 60 weight percent, or even 100 percent water based on the total weight of the liquid carrier. In most modes of practice in the coating industry, water-based compositions use water in combination with one or more organic solvents.
Because of the many benefits and advantages offered by phenolic resins, it would be desirable to use phenolic resin(s) in water-based latex compositions. A water-based latex composition refers to a composition that includes dispersed polymer particles, often sub-micrometer in size, in which the polymer particles are prepared via emulsion polymerization in a liquid carrier in which the continuous phase is aqueous. Incorporating phenolic content into latex compositions is not easy. Unfortunately, most phenolic resins are incompatible with water-based latex compositions. This makes it difficult to add sufficient phenolic resin content to existing latex compositions in a manner that allows the phenolic resins to be stably dispersed in the latex composition for long periods of time. Further, many emulsion polymerization processes are intolerant to the presence of phenolic resin content, because a phenolic resin tends to function as a chain terminating agent to prevent an adequate degree of polymerization. This makes it difficult to prepare a latex composition in situ with phenolic material already present. Accordingly, there is a strong demand for strategies that make it easier to incorporate one or more phenolic resins into water-based latex compositions.